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Connecting a Step Up Transformer

I've been reading Art Dudley's article in Stereophile regarding the Step Up transformers in the past two issue with great interest. I currently use a Shelter 901 (0.5mV) with an ARC PH3 SE. My preamp is the ARC LS25 MK I. With analog, I set the output at 24dB, but I still have to crank the volume all the way to the 12 o'clock or 1 o'clock position.

I've been thinking that a step up transformer might be the answer if I want to play with some other low output MC cartridges.

So I have the following questions:

1/ Currently the cartridge loading is set at 100 ohm on the PH3 per Shelter recommendation. If I add a transformer, does it mean I need to go back and change the loading on the PH3 back to 47k ohm?

2/ I take it that when you use a transformer, you will still have to match loading? So it is better to find one that allows you to adjust the loading?

3/ The PH3 only has 54dB gain. ARC recommended using MC cartridges with output higher than 0.6mV. So with a transformer, I should be able to resolve the gain issue, and also be able to use low output cartridges, ones with 0.3mV output, correct?

4/ What type of cable do you need to connect a transformer? I assume the phono cable w/ ground will just go into the transformer. How about from transformer to phonostage? Just a regular pair of RCA interconnects? Do I need a ground cable between the transformer and the phonostage? It would need to be the same length as the RCA cable?

FrankC
gundam91
7 responses Add your response
cleopatra52's avatar
cleopatra52
98 posts
 
It would be a good idea to read the Bent Audio site which contains good information on this subject: http://www.bentaudio.com/parts/tx103loadhow.html

1) The step-up ratio of the transformer will be important in determining the load the cartridge will see. For example, I am using a step ratio of 1:10 straight into the mm phono stage (47 Kohms). In this case, the cartridge will 'see' a load of 470 ohm.

2) Yes, you still have to match load and the better transformers, like the Bent Audio Mu-Mc, will allow you to do this. Again, the Bent Audio site does explain this.

3) Yes. However, you should be careful that you get the right transformer to match the gain of your phono preamp and the cartridge you plan to use. A transformer with too high a step up ratio could overload the phono preamp.

4) You are dealing with very weak signals that are sensitive to interference and RFI. The cable between the transformer and phono stage should be as short as possible and it is likely you will have to experiment with grounding. I run a simple ground wire from the Mu-Mc to my K&K phono stage and have no audible hum.

Hope this helps.

Terry
piedpiper
753 posts
 
I wouldn't bother degrading your sound in order to get your volume pot below 12 o'clock. You will need to set the loading to 47k and set the phono gain to the MM setting.
herman
2,259 posts
 
The closer the volume control is set to wide open the better off you are. If you are 12:00 to 1:00 then you have plenty of gain and it would be better if you could turn it up more, not less.

If you set the volume lower with your CD player then I would be more concerned about that.
gundam91 OP
506 posts
 
ARC LS-25 allows you to select output for each of the sources at 12dB, 18dB, or 24dB. The CD source is set at 18 dB gain at the preamp, and I only need to turn the volume to about 9 to 10 o'clock position to get the same volume. (As compared to vinyl which is set at 24dB gain, and volume would be turned to 12 o'clock/1 o'clock position)

The issue with turning the volume to 12 o'clock and 1 o'clock position is that the background is not as quiet.

FrankC
piedpiper
753 posts
 
a quiet step up such as a transformer will certainly give less noise, but at the expense of the added degradation of another stage and interconnects.
herman
2,259 posts
 
I think you are trying to solve a non-existent problem. The phono is not going to be as quiet as the CD no matter what you do. In between songs you will almost always hear some noise. If it doesn't interfere with the music then forget about it. Adding a transformer might make matters worse because they can pick up noise and hum themselves

I would try setting the gain to 12dB and make it up by turning up the volume control. The volume control is not adding noise, it is coming from somewhere before it, probably the phono stage and maybe because you selected more gain from the preamp.

If you can't turn up the volume control past 10:00 with the CD then you have too much overall gain and should select 12dB.
gundam91 OP
506 posts
 
Thanks for all the input. I will experiement with the volume and gain settings.

FrankC
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